Medical catheter

ABSTRACT

A medical device, such as a catheter in which the catheter has a relatively stiff proximal section and a relatively soft distal section joined by a relatively short transition section in which the materials of the proximal and distal sections are gradually combined in the transition section to form a continuous unbroken tube without abrupt joints. The relatively soft distal section can be provided with a balloon, and the catheter can be provided with multiple passageways. In addition, the catheters can be provided with braiding to provide the finished product with more torqueability and kink resistance.

This application is a continuation of application Ser. No. 08/460,662,filed Jun. 2, 1995, now U.S. Pat. No. 5,622,665, which is a division ofapplication Ser. No. 08/230,333, filed Apr. 20, 1994, now U.S. Pat. No.5,533,985.

BACKGROUND OF THE INVENTION

Catheters are used in the field of medicine in a variety of medical andsurgical procedures. For example, catheters are used extensively fordelivering diagnostic or therapeutic agents to a selected site withinthe body. Microcatheters are used in neurointerventional and similarprocedures. These catheters are commonly threaded through a vessel orartery and frequently follow a tortuous path in order to reach the sitewhere the agent is to be applied. A balloon catheter for treating, forexample, arterial stenosis has an inflatable balloon at its distal end.This catheter also follows a tortuous path to reach the site of thearterial restriction. The balloon is then inflated via a lumen throughthe shaft of the catheter, applying pressure to expand the stenosisagainst the artery wall.

Because these catheters often must be threaded through a tortuous path,the catheter must be rigid enough to allow the distal end of thecatheter to be manipulated by the physician or surgeon. On the otherhand, the catheter must be quite flexible to permit it to follow thetortuous path to the desired site of application. In order tosatisfactorily meet the requirements of flexibility and also stiffnessfor manipulation, various designs of catheters are known and used. Onesuch catheter utilizes a flexible catheter with an inflatable balloon atits distal end which, when partially inflated, will be carried by theblood flow to the desired location. Such catheters, however, cannot beused if the site where the agent is to be applied can be accessed onlythrough a vessel that has a low blood flow rate.

More commonly, guide wires are used which can be advanced to the site,and with the guide wire in place, the catheter can then be telescopedover the wire and advanced to the application site. Catheters that usethe guide wire technique, however, still must be sufficiently flexibleto track the wire and sufficiently rigid so that the catheter can beadvanced without buckling at the proximal end.

In order to overcome these limitations and difficulties, differentialstiffness catheters have been developed which have different degrees offlexibility throughout their length. These catheters have a long andstiff proximal section coupled to a short and soft or flexible distalsection that will track the guidewire. With these differential stiffnesscatheters, the physician or surgeon can push and maneuver the stiffproximal end to effectively advance the soft distal end. For example,Engelson U.S. Pat. No. 4,739,768 discloses a catheter which has arelatively stiff proximal segment and a relatively flexible distalsegment, the segments being formed by forming the proximal segment ofinner and outer coaxial tubes, one of which is relatively stiff, and theother of which is relatively flexible. The distal segment is then merelyan extension of the relatively flexible tube.

Because this type of differential stiffness catheter is usually made byhand by joining two or more pieces of tubing together, they are laborintensive and therefore expensive to manufacture. Moreover, thesecatheters tend to buckle and to kink at the joints where there occurs anabrupt change in stiffness. Buckling and kinking are very undesirablecharacteristics for catheters. Also, there is a tendency for the jointsto separate leaving the tip of the catheter inside the body andrequiring surgery to retrieve it. Attempts have been made to reduce thebuckling and kinking problems and prevent joint separation by making thecatheter with a relatively soft layer throughout the entire length ofthe catheter, but this construction results in reduced stiffness at theproximal end.

Prior art patents such as Quackenbush U.S. Pat. No. 5,125,913 and FlynnU.S. Pat. Nos. 4,250,072 and 4,283,447 recognized some of the potentialbenefits of using a process technology called co-extrusion to makevariable stiffness catheters. However, disappointing results have beenobtained in following their teachings. Co-extruded catheters produced byperiodic interruption using prior art teachings result in undesirablylong transition sections, which are the sections of the catheter wherethe tubing changes from a stiff tube to a soft tube. Some of thecatheters produced by these prior art processes have transition sectionsthat extend the entire length of the catheter. These undesirably longtransition sections have been the major problem in attempts to makecatheters and other medical tubing with interrupted layers orinterrupted elements. Also, the interrupted layers co-extrusion processresults in only a moderate difference in stiffness between the proximaland distal sections--less than is considered desirable for catheters.Moreover, since very long cycle times are required for known interruptedlayer processes, these co-extrusion processes are not as economicallyfeasible as first thought. Further study has shown that thesedeficiencies cannot be corrected by simple means, such as processvariable changes, but rather require fundamental changes in the processitself.

Furthermore, the prior art does not recognize the possibility of forminga very secure joint between soft and stiff resins by using co-extrusionand sequential extrusion processes to produce a "wedged-in" transitionsection in which one resin is securely locked or wedged into anotherresin.

In addition to the foregoing, medical catheters must have the proximalend attached to a variety of different connectors which facilitateattachment of one or more medical device or devices necessary to carryout the particular medical procedure using the catheter. At the point ofattachment of the catheter tube to the connector, kinking can easilyoccur and restrict the flow of the fluid being introduced into thecatheter tube. To minimize the probability of kinking, prior artcatheters commonly use a short length of a flexible rubber tube thatextends from the connector and into which the proximal end of thecatheter is inserted and affixed. Although use of this rubber tubereduces kinking of the catheter tube, there is still considerable strainapplied to the catheter tube at the point where it exits the rubber tubeconnector. This kinking problem also exists in tubing used in nonmedicalapplications.

It is therefore an object of the invention to develop a co-extrusionmethod of forming tubing such as catheters in which the transitionsection can be shortened and controlled to the point where a variety ofsuitable medical catheters with interrupted layers and elements can beproperly produced at a reasonable cost.

It is another object of the invention to develop a method of forming the"wedged-in" construction in the transition section of the tubing toproduce an extremely secure joint between different resins.

It is yet another object of the invention to use the methods of theinvention to provide an improved strain relief section where the tubingis joined to a connector or other device thereby eliminating kinking atthis joint.

It is a further object of the invention to develop a suitableco-extrusion head and system to carry out the methods of the invention.

SUMMARY OF THE INVENTION

The invention relates to the manufacture of a tubing having differentproperties, for example differential stiffness, in different sectionsalong its length. Such tubing is useful in many medical applications,such as catheters. A catheter made utilizing the principles of theinvention has a stiff proximal section for "pushability", a flexible,soft distal section for tracking the guidewire and a unique transitionsection of controlled length, e.g., significantly shorter than ispossible with prior art extrusion fabrication methods, in which thestiff material of the proximal section and the flexible material thatforms the distal section are sequentially co-extruded, e.g., "wedgedinto" one another to produce an extremely secure, practicallynon-breakable joint between the two materials. The merging of the twomaterials is very smooth and gradual to eliminate the buckling andkinking that usually occurs at abrupt joints between two materials ofdifferent stiffness. The gradual transition also facilitates tracking ofthe guidewire and is short enough to be useful in catheter applications,including so-called microcatheters. Typically, the average length of thetransition section in such medical catheter tubing is about 0.25-20inches, preferably about 0.5-10 inches.

Tubing produced according to the invention for medical applications isextremely small in diameter and wall thickness and cannot be producedpractically by hand. Therefore, in another aspect of the invention, alow cost automatic co-extrusion process and co-extrusion head areprovided. The co-extrusion head is designed to minimize volumes for allthe flow channels, within limits discussed further below, and has nomoving parts which is essential to precisely control the diameter of thetubing and to assure consistently high quality of the final product. Thedesign of the co-extrusion head also provides for virtually mutualexclusivity of resins, i.e., it is possible to produce a tubing that hasvirtually 100% of the stiff resin in the proximal section and virtually100% of the soft resin in the distal section. Moreover, the method andsystem of the invention provide for the production of tubing usingseveral thermoplastic resins of varied stiffness which can beautomatically fed into the co-extrusion head in a precisely synchronizedfashion to produce a tubing having different resins or resincombinations in different sections of the tubing, always with gradualtransitions from one to the other in short transition sections.

All of the foregoing, as well as other features of the invention, willbecome more readily apparent from the detailed description of thepreferred embodiments of the invention as illustrated in conjunctionwith the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A, 1B, 1C, 1D, 1E and 1F are longitudinal cross-sectional viewsof a portion of differential stiffness catheters of the type to whichthe invention relates;

FIG. 2 is a schematic diagram of a system for producing a differentialstiffness catheter using the co-extrusion technique of the invention;

FIGS. 3A and 3B are views, partly in section, of a two-stage flowmodulator used in the system of the invention, FIG. 3A being a frontelevational view and FIG. 3B being an end elevational view;

FIG. 4 is a side elevational view partly in cross-section illustrating athree-layer co-extrusion head utilizing the principles of the invention;

FIG. 5 is an end view of the co-extrusion head of FIG. 4;

FIG. 6 is side elevational view partly in cross-section similar to FIG.4 but showing a two-layer co-extrusion head;

FIG. 7 is a series of diagrams showing the steps of a process performedusing the principles of the invention;

FIGS. 8A, 8B and 8C each contain various views of a section of acatheter to illustrate the differences between a slanted end and an evenend, and the configuration normally found in an even end;

FIGS. 9A and 9B each contain various views of a catheter end toillustrate the differences between a layer construction and an elementsconstruction;

FIG. 10 is a longitudinal sectional view of another type of tubing thatcan be produced using the principles of the invention;

FIG. 11 is a longitudinal sectional view of still another type of tubingthat can be produced using the principles of the invention;

FIGS. 12A-12F show a multi-lumen catheter tubing produced using theprinciples of the invention;

FIG. 13 is an angiographic catheter produced using the principles of theinvention;

FIG. 14 is a strain relief joint including tubing produced using theprinciples of the invention;

FIG. 15 is a soft tip guiding catheter produced using the principles ofthe invention;

FIG. 16 is a catheter guidewire coated with tubing produced using theprinciples of the invention;

FIG. 17 is a metal braid reinforced tubing produced using the principlesof the invention; and

FIG. 18 is a wire wound tubing produced using the principles of theinvention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE INVENTION

In order to fully and completely understand the co-extrusion andsequential extrusion of differential stiffness tubing, certain terms andphrases that are used herein must be clearly understood. The terminologyused herein is that commonly used and understood by those ordinarilyskilled in the art unless otherwise indicated or as modified by thespecific definitions set forth in this specification. When the terms"outside layers" or "inside layers" are used herein, these refer to theinside layer and outside layer of the tube, and sometimes these arereferred to simply as the "side layers". The "interior layers" are allthe layers that form a tube other than the side layers.

Also, as used herein, the word "elements" means any shape that is notcontinuous in the cross-sectional direction of a tube, such as theconstruction illustrated in FIG. 9B. The cross-sectional shape of theelements can be round as shown, or they can be rectangular or any othershape.

In FIGS. 1A, 1B, 1C, 1D, 1E and 1F, there are illustrated differentialstiffness catheters of the type to which the invention relates. Suchcatheter tubing is typically of an outside diameter of about 0.25-0.02inch. The tubing and process described herein are most valuable forcatheters no larger than 0.08 inch outside diameter. In each of thesedrawing figures, the diameter has been greatly enlarged and the lengthcompressed so as to clearly show the different layers between thecatheter walls. The catheters shown in FIGS. 1A, 1B, 1C, 1D, and 1F eachhave an inner wall 10 and outer wall 12 that define an annular tube witha longitudinally or axially extending passageway 14 through which aguidewire (not shown) can be passed and through which fluid flows. As iswell known to those skilled in the art, the catheter has a distalsection 16 which is commonly the soft or flexible portion of the tubeand a proximal section 18 which is the stiff portion of the tube. Asshown in FIGS. 1A-1F, the "transition section" 19 is the length of thecatheter in which the tube changes from a stiff tube to a soft tube. Theterm "transition section is defined further below.

A key feature of the invention is the gradual change and the controlled,shorter length of the transition section between the soft, flexibleportion and the stiff portion of the tube. Another key feature of someaspects of the invention is the "wedged-in" construction in thetransition section 19 of the catheter where a layer of one materialforms a wedge-shaped profile extending into another material. Thisconstruction is naturally formed provided that the "skewing volume",(defined hereinafter) is not overly short, and the viscosity of the"wedging" material or resin is not overly high when compared with theresin into which it is "wedged". As will be more clearly explainedhereinafter, in the co-extrusion process, the speed of flow of the resinis usually the greatest near the center of any flow channel of theco-extrusion head and slowest near the walls of the flow channel.Therefore, any new and different material introduced into one flowchannel tends to flow out first near the center of the channel and lastnear the walls, thus forming the "wedged-in" structure. Another way offorming a "wedged-in" construction is to introduce an interior layer ata gradually increasing rate.

In practicing the invention, one material or resin is always graduallycombined with another material in the transition section 19, in someaspects of the invention forming a "wedge" structure. The "wedge" can bein the form of a gradually thinning layer, as shown in FIG. 1A, or inother gradually changing shapes, such as multiple spear points. Aspreviously mentioned, this wedge construction forms an extremely secure,virtually unbreakable joinder between two resins because of the largesurface contact area created between the two resins which effectivelyrestricts relative movement between them.

The invention is illustrated in connection with examples of cathetersthat have differential stiffness because of the use of differentmaterials or resins. FIG. 1A shows a catheter in which the proximalsection 18 is of a single layer of material that makes the section 18stiff. The distal section 16 is also of a single layer of soft materialthat makes the distal section 16 soft and flexible. In the transitionsection 19 the stiff material of the proximal section 18 is wedged intothe soft material of the distal section 16, thus providing an enclosed,secure joinder of the stiff and soft materials and avoiding an abruptchange in material that can cause kinking.

FIG. 1B shows a catheter in which the stiff material of the proximalsection 18 is an inside layer 15 with the soft material of the distalsection 16 forming an outside layer 11 along the proximal section 18 aswell as the transition section 19. As in the embodiment of FIG. 1A, thestiff material is wedged into the soft material in the transitionsection 19.

In FIG. 1C, the construction of the catheter is similar to that of theembodiment of FIG. 1B except the stiff material also extends into thedistal section 16 to form a thin inside layer 13 with the soft materialforming the outside layer 11 as in the embodiment of FIG. 1B.

FIG. 1D shows a catheter construction in which the stiff material of theproximal section 18 forms an interior layer 17, and the stiff materialextends into the transition section 19 and is wedged into the softmaterial of the transition section 19. However, in this embodiment, theoutside layer 11 is uninterrupted and extends the entire length of thetubing from the proximal section 18 through the transition section 19and the distal section 16. Also, in this embodiment of FIG. 1D, theinside layer 13 is of a different material from the material of eitherthe outside layer 11 or the interior layer 17, layer 11 being of amaterial that is softer than the stiff material but suitable formovement of a guide-wire.

FIG. 1E shows a catheter construction in which a plurality of materialsof different stiffness provide a differential stiffness catheter. Thestiffest material of the proximal section 18 forms an interior layer 15,and the stiffest material extends into the transition section 19 and iswedged into the less stiff material in portion 19c of transition section19. This less stiff material, in turn, is wedged into a softer materialin portion 19b of transition section 19, which, in turn, is wedged intothe softest material in portion 19a of transition section 19. Thus, inthis embodiment, four materials of different stiffness provide thedifferential stiffness along the length of the catheter. However, threeor more than four materials may also provide a similar construction. Inthis embodiment, both the outside layer 11 and the inside layer 15 areuninterrupted and extend the entire length of the tubing from theproximal section 18 through the transition section 19 and the distalsection 16. In the embodiment of FIG. 1E, the four different materialsare successively wedged into one another in such a way that portions19a, 19b and 19c of transition section 19 overlap and three materialsare present in each area of overlapping portions. Alternatively,portions 19a, 19b and 19c may be immediately adjacent to or separatedfrom one another within transition section 19 to provide differentialstiffness for the catheter.

FIG. 1F illustrates a non-wedged construction in which the stiffmaterial and the soft material both extend the full length of thecatheter, the differential stiffness resulting from a change in relativethickness of the two materials which takes place in the transitionsection. The stiff material forms an inside layer 15 which issignificantly thicker in the proximal section 18 than in the distalsection 16, gradually decreasing in thickness in transition section 19.The soft material forms an outside layer 11 which is significantlythinner in the proximal section 18 than in the distal section 16,gradually increasing in thickness in the transition section 19. Thusinner layer 15 provides a continuous smooth surface for passage of aguidewire, while outer layer 11 provides a low-friction layer forpassage of the catheter through bodily passages. Typically, transitionsection 19 in such a catheter is about 0.25 to 20 inches long. Inalternate embodiments, either inner layer 15 or outer layer 11 may notextend the entire length of the catheter; that is, inner layer 15 mayterminate anywhere along the length of proximal section 18 or outerlayer 11 may terminate anywhere along the length of distal section 16.Typically, however, either inner layer 15 or outer layer 11 or bothextend the entire length of the catheter. Alternatively, three or morematerials may extend along the length of the catheter in a non-wedgedconstruction. For example, a third material may provide an interiorlayer of intermediate stiffness between the stiffer layer and the softerlayer. This interior layer provides a relatively thick lengthwise layersegment in transition section 19 between the thicker stiff layer ofproximal section 18 and the thicker soft layer of distal section 16,gradually decreasing in thickness in the proximal and distal sections.

Generally, as used herein, the term "transition section" is known in theart, and refers to the portion of the catheter in which the propertiesof the tubing change from those principally provided by one material,i.e., the primary material of the distal section (distal primarymaterial) to those principally provided by another material, i.e., theprimary material of the proximal section (proximal primary material).Along the length of the tubing, the volume percent of the proximalprimary material changes from a maximum in the proximal section to aminimum in the distal section, while the reverse is true for the distalprimary material. The transition section, therefore, may be definedrelative to this maximum volume % of the proximal primary material(Vmax). That is, as used herein, the term "transition section" isdefined as the portion of the tubing between two points along the lengthof the tubing. The proximal primary material at the proximal end pointof the transition section is at least about 95% of the Vmax, while theproximal primary material at the distal end point of the transitionsection is no more than about 5% of the Vmax. Thus, for the simplecatheter tubing of FIG. 1A, the Vmax is 100%, and the transition sectionis the portion between about 95% (95% of 100%) and about 5% (5% of 100%)by volume of the proximal primary material. In FIG. 1D, the outsidelayer of the distal primary material carried throughout the length ofthe tubing does not affect the definition of the transition layer, i.e.,if the Vmax (of the proximal primary material) is 80%, the transitionsection is the portion between about 76% (95% of 80%) and about 4% (5%of 80%) by volume of the proximal primary material. In FIG. 1C theproximal primary material is carried into the distal section as aninside layer throughout the catheter tube. Therefore, for the purpose ofdetermining the transition section boundaries, this layer may be treatedas a separate material not part of the Vmax. Thus, if the true volume %of the proximal primary material in the proximal section is 95% but theinside layer alone is 5%, then the Vmax is about 90%, and the transitionsection is the portion between about 85.5% (95% of 90%) and about 4.5%(5% of 90%) by volume of the proximal primary material.

As previously indicated in describing the background of the invention,the primary difficulty encountered in co-extrusion of differentialstiffness tubing as taught by the prior art is the length of thetransition section 19. As will be evident from further description ofthe invention, there are a number of design aspects of the prior artco-extrusion heads and systems that result in these undesirable longtransition sections.

Referring now to FIG. 2, there is shown schematically a system forco-extruding differential stiffness tubing, such as catheters. Thesystem includes a co-extrusion head indicated, generally, by thereference numeral 20 into which extruders 22, 24 and 26 feed thedifferent resins, such as a soft resin and a stiff resin, that will beused to form the finished tubing. For purposes of illustration, extruder22 provides a resinous stream for a resin "A" which, for example, willultimately form the outside layer 11 of the catheter of FIG. 1D, whileextruder 26 provides a stream of resin "B" that will form the interiorlayer 17 of the finished catheter. Similarly, extruder 24 provides aresinous stream for resin "C" which is the material that will form theinside layer 13 of the finished catheter. As illustrated in FIGS. 3A and3B, and as more fully described hereinafter, a modulating device,indicated generally by the reference numeral 28, regulates the flow ofthe resins from each of the extruders 22 and 26 into the co-extrusionhead 20, while another modulator 27 may be used to bleed resin "A" fromthe head 20 to relieve residual pressure. To produce catheter tubingwith differential stiffness, the modulators 28 are actuated periodicallyand in synchronized fashion to abruptly stop or change the resin flow tothe head 20. Because of the design of co-extrusion head 20, theinterface between the stiff resin and soft resin is naturally shearedand elongated when flowing through the flow channels of the head 20.Thus, these abrupt changes or stoppages by the modulators 28 result in avery gradual change of stiff layer thickness in the tubing, creating thegradual stiffness change and resulting in the wedge structure in thetransition section 19 of the catheter tubing. After discharge from thehead 20, the tubing is cooled by passage through a water tank 21, alaser mike 23, puller 25 and cutter 27 to form the catheter tubing.

As is well known to those skilled in the art, a co-extrusion head is anassembly of many precision machined parts that provide a plurality offlow channels, each of which is connected to one of the extruders. FIG.4 illustrates the design of a co-extrusion head 20 for producing atubing of three different materials, such as that shown in FIG. 1D. FIG.6 illustrates a head design for producing a tubing of two differentmaterials, such as the tubing illustrated in FIGS. 1A, 1B and 1C. Theco-extrusion head 20 has a main body 30, usually of a cylindrical shape,in which is formed an inlet 32 for resin "A", an inlet 34 for resin "B"and an inlet 36 for resin "C". Each of the inlets is connected to therespective one of the extruders 22, 24 and 26, and through a flowchannel formed in the main body 30, to a die 40 which, together with thetip 42, forms the exit from the head 20 for the flow of resins to createthe desired configuration of the finished product. As illustrated inFIG. 4, the co-extrusion head 20 has a flow channel 38 connected toinlet 32 for resin "A", a flow channel 44 that is connected to inlet 34for resin "B" and a flow channel 46 that is connected to inlet 36 forresin "C". A removable cap 48 on main body 30 provides access to thehead to change the die 40 and tip 42. It is not an unusual situationthat a producer of medical tubing such as catheters will frequentlychange the die 40 and tip 42 without changing the entire co-extrusionhead 20 in order to produce tubing of different sizes or of differentmaterials. This change sometimes can take place several times a day. Toaccommodate this need, the design of the head 20 can be such so as toinclude in the same co-extrusion head 20 both large and small dies 40and tips 42.

When used herein, a "side stream" means any resinous stream in theco-extrusion head 20 of the type of FIG. 4 that is either on the outsideor on the inside at point 51 where the resinous streams meet prior toexiting the head 20 at point 52. Similarly, "interior stream" is anyresinous stream in the co-extrusion head 20 that is between two sidestreams at point 51.

"Contact volume" of any resin, such as resin "C", is the volume in theflow channel portions of the co-extrusion head 20 where resin "C" isflowing jointly with at least one other resin, such as resin "B", andwhere resin "C" is in intimate contact with solid non-moving surfaces ofthe head 20 in the section where there is such joint flow. Also, theterm "contact surface" means the solid, non-moving surface of head 20with which resin "C" is in intimate contact in the "contact volume". InFIG. 4, contact volume for resin "C", as well as resin "A", is thevolume in the flow channel between point 51 and exit point 52. Contactvolume for resin "B" is zero, since it does not contact any solidsurface after it passes point 51.

Also, by definition, the "residual flow volume of a resin" is the volumeof a flow channel section between the discharge from the modulator 28and the point where the resin joins another resinous flow for the firsttime.

The "skewing volume" of a resin is the volume in a flow channel betweenthe point where the resin stream "C" joins another flow stream for thefirst time (the start of the contact volume) and the point where theresin exits from the co-extrusion head 20. In the typical co-extrusionhead example of FIG. 4, the "skewing volume" of all three resins is thevolume in a flow channel between point 51 and the resin exit point 52.

As previously indicated, the contact volume and the skewing volume areprobably the most influential factors in forming the length of thetransition section. The lower the volumes, the shorter the transitionsection that will be formed. The undesirable able lengthy transitionsection occurs in prior art co-extrusion heads because the solid contactsurface drastically slows down the resin flowing by it to cause a"drag-out" effect that overly elongates the resin interface. For theshortest transition section, zero contact volume should be designed intothe co-extrusion head 20 since zero contact volume design completely orsubstantially eliminates the drag-out effect of the interruptable resin"B", and as a result, this design will produce the shortest transitionsections possible when the flow of resin "B" is interrupted. However, asa practical matter, for resin "A", FIG. 4 illustrates a flow channelarrangement in a co-extrusion head design that we have termed a"die-length contact volume" head design. With this design of FIG. 4, thecontact volume is no greater than the volume covered by the length ofthe die 40 multiplied by a factor of 10, and is no less than the volumecovered by the die length. Preferably, the contact volume is equal tothe volume covered by the die length. This "die length contact volume"head design is especially useful in interrupted side streams inco-extrusion rather than where the center streams are interrupted, andin some designs, the use of a die is deemed necessary to control theoverall tube concentricity and assure its uniformity. As a practicalmatter, in many cases the contact volume should not exceed the volumecontained in the desired transition section length multiplied by afactor of 10. For producing some catheters of a small diameter, thecontact volume can be as small as 0.5 ml. Thus, by the die-lengthcontact volume design of the co-extrusion head 20 as shown in FIG. 4 forresin "A", one of the most influential causes of lengthy transitionsections is practically eliminated.

Also, it is desirable to keep the discharge opening of the flow channel44 for resin "B" as small as possible where it joins the flow channel 38for resin "A". As previously described, the design of the head 20 shownin FIG. 4 uses a die-length contact volume for resin "A", and bychanging the thickness of the die 40, the contact volume can be varied.

The length of the transition section also can be changed by a simplechange of tooling in the tip 42 and die 40 of the head 20. By changingthis tooling, both the skewing volume for resins "A" and "B" and thecontact volume for resin "A" can be modified.

However, there are other factors that influence the length of thetransition section 19 in a differential stiffness catheter. One factorthat influences the length of the transition section is the finalcross-sectional area of the tubing to be formed. Tension and internalair pressure are applied, as is known in the art, to the hot,as-extruded tubing exiting the extrusion head, resulting in elongationand shaping of the tubing to preselected inside and outside diameters.The length of the transition section is inversely proportional to thefinal cross-sectional area of the wall of the tubing. In other words,thin tubing with very small diameters tend to have very long transitionsections.

As is well known to those skilled in the art, in any continuousextrusion process, when the resin flow to the extrusion head is suddenlyshut off, a small amount of resin will continue to flow for some time,even for minutes in some instances. Theoretically, this "residual flow"effect on the interrupted resin stream "B" will therefore cause alengthy transition section. However, we have determined that the amountof residual flow decreases with a decrease in the residual flow volumepreviously defined herein. Therefore, by keeping residual flow volumesmall, the effect of residual flow can be minimized. Another way toreduce the residual flow effect is through the use of modulators.

As is well known to those skilled in the art, after a resin stream, suchas resin "B" is introduced into a stream of resin "A" as illustrated inFIG. 7, the shape of the resin stream "B" changes as it flowsdownwardly, and this results in a longer transition section. The reasonfor this phenomenon is that fluids flow faster at the center of a streamthan at the sides, and therefore, the further the fluid flows, the more"skewed" becomes the stream. In the case of the flow of resin "B", thisresults in a longer transition section. Therefore, by keeping theskewing volume to a minimum, preferably below the resin volume containedin the desired transition section multiplied by a factor of 10, thelength of the transition section can also be kept to a minimum.

The length of the transition section can also be changed by changing theviscosity of the resins. For example, when the leading edge of the stifflayer is used to wedge into the layer of softer material as illustratedin FIGS. 1A, 1B, 1C, 1D and 1E, raising the viscosity of the stiff layeror lowering the viscosity of the softer side layers will shorten thelength of the transition section.

Note that from the design of the flow channels in the head 20 asillustrated in FIGS. 4 and 6, all of the foregoing factors have beendesigned into the head 20 so as to result in keeping the transitionsection 19 to a desired short length. Another important feature of thehead 20 of the invention is the use of means to adjust the "slanting" ofthe "wedge" of the stiff layer. In FIGS. 4 and 6 the head designincludes adjustment screws 54 on the rear of the head 20. By adjustingthe screws 54, the length of the annular stiff layer, and thus thelength of the transition section 19, can be varied along only a portionof the annular layer to produce a slanted end, as illustrated in FIG.8A. FIG. 8B shows the stiff layer of uniform length around the entirecircumference to produce an "even" wedge. In FIGS. 8A and 8B, thetubings are shown schematically; in particular, the downstream, thinnerends of the wedges of the stiff layers are shown as sharp edged andsmooth. In the actual product produced by the process described above,however, the downstream edge of the wedged-in layer ends in a pluralityof longitudinally extending "spear points" of one material extendinginto the other in the wall of the transition section, as shown in FIG.8C. (In the tubing of FIGS. 8A-8C, points of the stiffer material extendinto the softer material.) The downstream ends of the points arecircumferentially spaced apart in the wall of the transition section,and the points gradually increase in size in the upstream directionuntil they join to form the annular layer shown in FIGS. 8A-8C.

Also, as is evident to those skilled in the art that in making tubingfor catheter applications, it is very important to have precise controlover the inside and outside diameters of the tubing as well as the wallthicknesses. In any tube extrusion process, some variation in tubediameter (either interior or outside diameter) always occurs if thetotal resin flow rate is changed. The variations in diameter also occurwhen total resin flow is reduced due to the interruption of one or moreresin streams in co-extrusion processes. The extent of the diameterchange increases with the increased percentage of the stream of theinterruptable resin relative to the total resin flow. Therefore, toosudden an interruption, although good for producing short transitionsections, can sometimes produce too sudden a change in the diameter ofthe tubing and result in undesirable ripples. Also, because there isalways a lag between interruption induced diameter change and thetransition section of a tube, this lag distance can be as short as afraction of an inch or as long as many feet. The lag distance increaseswith the increase of skewing volume and it is inversely proportional tothe final cross-sectional area of the tubing.

An important design feature of the invention is that the co-extrusionhead 20 of the invention has no moving parts, thus assuring that thediameter of the tubing will be consistently accurate which is especiallyimportant for medical tubing such as catheters. Also important inmaintaining consistent diameters of the tubing is the close match ofmelt strengths of the resins used.

Although most of the emphasis in the system of the invention has been onthe design of the co-extrusion head 20 in order to produce tubing of aconsistent diameter and having a transition section of a desired shortlength, the modulator 28 is also important in order to regulate andcontrol the actual length of the transition section as well as theprofile for any particular tubing. The modulator 28 functions toregulate the flow rate of the resins in a rapid and precise fashion. Atypical modulator 28 is illustrated in FIGS. 3A and 3B, which show atwo-stage type modulator in which the flow can be regulated so as toflow through to the head 20 or interrupted to direct the resin flow to arecycling container (not shown). The modulator 28 must be fast acting soas to operate to change the flow within no more than two seconds andpreferably in less than 0.5 seconds. The modulator 28 thus has an inlet56 leading to a chamber 57 into which extends a valve member 58 operatedby actuator 60. Depending upon the position of valve member 58, theresin flowing into the chamber 57 is directed to the outlet 62 which isconnected to the head 20 or the resin is directed into a bypass outlet64. A variety of different designs of the modulating device 28 can beutilized as long as they have extremely fast responses and can producerather precise flow controls. Since a typical cycle for making onelength of tubing for a single catheter is only about 0.5 to 10 seconds,rapid response is extremely necessary. If desired, the modulator 28 canbe replaced by a plunger type modulating device, actuated by a servovalve with a programmer (not shown). The programmer may be of anysuitable design such as that marketed by "Moog" Electronics and SystemsDivision which produces and markets a line of parison programmingsystems.

It should be noted that if the co-extrusion head 20 is properly designedand operated, rather simple on-off types of modulating devices may besufficient, depending upon the type of tubing being produced. Also,devices with mechanical gradual flow reduction or gradual flow-increasefunctions can also be used depending upon the tubing requirements.

Although modulators may be programmed to deliver tubing having aconsistent diameter throughout the production cycle, small variations dooccur due to slight mismatches of the modulators. A tubing can beproduced with the most consistent diameter, e.g., for use as a catheter(before tapering) by keeping the skewing volume greater than the resinvolume contained in distal section 16 of the catheter, and preferablygreater than the resin volume of the distal section multiplied by afactor of 5.

In FIG. 7 there is illustrated the operating steps of a complete cyclefor producing the differential stiffness catheter of FIG. 1D. Thisfigure shows schematically the die 40 and tip 42 of the head 20 andillustrates the flow of the resins through the die 40 and tip 42. In thefirst phase, the flow of resin "B" is stopped while the flow of resins"A" and "C" are on. This forms the soft distal section 16 from resin "A"with a thin inside layer 13 formed of the material of resin "C". Theflow of resin "B" is then commenced while the rate of flow of resin "A"is reduced and the flow of resin "C" continues. In this phase, thetransition section 19 is formed. In the third phase, the flow of allthree resins is on to form the stiff, multi-layered proximal section 18.In the last phase, the flow of resin "B" is stopped and the rate of flowof resin "A" is increased to purge out resin "B". The cycle is thenrepeated starting with the first phase. Note that in all phases the flowof resin "C" is continuous and at a constant rate. This illustration ofa complete cycle of a process performed according to the principles ofthe invention shows that the process extrudes different resinssequentially as well as simultaneously. Prior art processes teach onlysimple co-extrusion in which different resins are extrudedsimultaneously only.

Although in many cases of differential stiffness tubing only one stiffresin will follow one soft resin in a consecutive manner, there areapplications in which three or four resins of different stiffness areco-extruded consecutively to produce a finished product with improvedkink-resistance, and with such a process, better control of tubingdiameters can be achieved.

The head and systems designs of the invention are also useful where itis desired to obtain a specified diameter and wall thickness and tocombine interrupted resin streams technology with tapering and lumen airregulating techniques. When so doing, the timing of all these devicesshould be synchronized to avoid drift.

It is also known to those skilled in the art that in co-extrudedproducts, a side layer of non-sticking, non-compatible material can beremoved in a post-extrusion operation resulting in a product with oneless layer. When this technique is used on tubing made with the methodsand systems described herein, this technique can be beneficial inreducing diameter fluctuations due to resin melt strength differences aswell as other reasons.

There are some applications where it is desirable to increase thetransition section in a medical tube, such as a catheter. The obviousremedy is to use the modulating device 28 to lengthen the transitionsection. However, as previously described, it is also possible toincrease the contact volume and/or the skewing volume in order tolengthen this transition section.

From the foregoing description it will be evident that we have describeda new technology for making tubing, especially medical tubing for suchapplications as variable stiffness catheters, soft tip catheters, etc.However, multi-lumen catheters can also be made using the principles ofthe invention. A typical multi-lumen tubing for such a catheter is shownin FIGS. 12A-12F. In FIG. 12A, tubing 100 includes stiffer proximalsection 101, transition section 102, and softer distal section 103. FIG.12B illustrates a cross section of softer distal section 103 taken alongline B--B of FIG. 12A. In FIG. 12B, distal section 103 includes lumens104 formed within softer material 105. FIGS. 12C-12E illustrate crosssections of transition section 102 taken along line C--C, D--D, andE--E, respectively, of FIG. 12A. In FIG. 12C, the portion of transitionsection 102 near distal section 103 includes lumens 104 formed withinsofter material 105, with inserts of stiffer material 106 on either sideof lumens 104. FIG. 12D, from near the center of transition section 102,is similar to FIG. 12C, but with larger insertions of stiffer material106. In FIG. 12E, the portion of transition section 102 near proximalsection 101 is made up largely of stiffer material 106, with thin layersof softer material 105 adjacent lumens 104 and the outer surface oftransition section 103. In FIG. 12F, proximal section 101 includes onlystiffer material 106, with lumens 104 formed therewithin. Typically,when a plurality of stiffer sections (each of which may form one or twoproximal sections 101) and softer sections (each of which may form oneor two distal sections 103) are extruded, a small remnant of stiffermaterial remains at the core of the softer and transition sectionsbetween stiffer sections. Such a remnant is shown in FIGS. 12B, 12C, and12D as remnant 107 of stiffer material 106.

Alternatively, the multi-lumen catheter may include three or morelumens, and the lumens may have any of a variety of shapes and may be ofthe same or different shapes and sizes. Such multi-lumen tubing isuseful for applications in which known multi-lumen-tubing has been founduseful, for example, in balloon catheters or electrophysiology (EP)catheters. A particular advantage is provided by the variable stiffnesstubing in an EP catheter. When the distal tip of the EP catheter isdeflected for maneuvering of the catheter through tortuous anatomies,the short transition section and soft distal section permit fasterrecovery of the straight-line axial configuration than has been achievedin prior art EP catheters.

Also, in addition to the two and three resin systems described herein,more than three resin systems can be made, such as for channel balloonconcepts and for some multi-lumen concepts. It should also be pointedout that the differential stiffness tubing made according to theinvention not only can be used for making the full length of thecatheter, they can also be used to make only a part of the catheter. Forexample, the invention can be used to make a single lumen tubing insidethe balloon, or produce a catheter with the distal section combined witha proximal section formed of either a braided construction or a metaltubing such as Nitenol tubing. Another type of catheter construction towhich the principles of the invention can be applied is to produce acatheter with a low friction layer on the inside surface for goodguidewire movement.

In yet another type of catheter construction, the catheter distalsection may be shaped, e.g., by heating to form a bent configuration inits relaxed state. For example, a J-tip or hook-shaped profile may beformed at the distal tip of the catheter. The wall of this bent or othercatheter may be perforated by known means, e.g., at the proximal sectionto provide for inflation of a balloon via the central passageway or atthe transition or distal section to provide for dispensing of fluidmedication or a fluoroscopic dye. An example of such a catheter is shownin FIG. 13, showing angiographic catheter 110 having proximal section111, transition section 112, and distal section 113. Distal section 113has been, e.g., heated and bent to form loop 114 in its relaxedconfiguration. In use, catheter 110 is threaded onto a guidewire (notshown), which holds distal section 113 in a configuration generallycoaxial with the guidewire. When the distal section 113 is in positionwithin a bodily passage, the guidewire may be partially withdrawn topermit distal section 113 to form loop 114 of its relaxed configuration,which holds catheter 110 in place. The wall of transition section 112may be perforated to provide apertures 115 for fluid communicationbetween central passageway 116 of catheter 110 and the outside of thecatheter. Apertures 115 may be used for, e.g., the dispensing of a dyefor fluoroscopic viewing of the bodily passage.

In addition to differential stiffness catheters, the invention can beemployed to produce soft-tip catheters, catheters of varied colors andfor "strain-relief" of any part of a catheter. In the last mentionedcase, a short differential stiffness section of the catheter is formedat the proximal end to provide a transition from the connector, forexample, to which the tubing is attached and the main portion of theproximal end of the catheter. Alternatively, a separate short piece ofdifferential stiffness tubing can be used to provide improved strainrelief to a known tubing/connector assembly. A strain relief insert ofthis type is shown in FIG. 14. Catheter 120 of FIG. 14 includes flexibleshaft 121 and rigid fitting 122 coaxial with shaft 121, shaft 121extending proximally and distally from fitting 122. Tubular strainrelief insert 123, which is a length of differential stiffness tubing,jackets flexible shaft 121 in the area of joint 124 between shaft 121and fitting 122. Portion 125 of stiff second section 126 of strainrelief insert 123 is disposed between shaft 121 and fitting 122, whilethe remainder of second section 126, transition section 127, and softfirst section 128 of insert 123 extend distally about shaft 121,providing graduated flexibility to joint 124 to prevent kinking of thejoint.

A typical soft tip guiding catheter is shown in FIG. 15, in which softtip guiding catheter 130 includes proximal section 131, transitionsection 132, and distal section 133. Transition section 132 has been,e.g., heat treated to form S-shaped portion 134 for maneuverability ofthe catheter, while short distal section 133 provides low-trauma softtip 135 for the catheter. Typically, stiffer proximal section 131 hasbeen reinforced with a metal braid jacket (not shown) embedded in thewall of proximal section 131. Such an embedded metal braid jacket isdescribed further below.

The invention, however, is not limited to catheter products but can alsobe employed in producing other types of tubing and rods that requiresections of varied properties. For example, FIG. 10 illustrates "bump"tubing in which the insert end is of stiffer material and the bell endis of soft material. Unlike bump tubing produced using prior art methodswhere both ends are soft, the stiff insert end of the tubing produced bythe invention provides for a more secure and tighter fitting connection.Also, the invention can be used to produce a new tubing for quickconnect fittings in which the ends are of stiff material but alternatingsections are of a softer material to provide flexibility for the lengthof tubing between the fittings while the stiff ends are easier to fitinto quick connect fittings. A tubing of this type is shown in FIG. 11.The tube includes several stiff first sections, each pair of firstsections having a soft second section therebetween. Each stiff sectionis joined to an adjacent soft section by a transition section to form acontinuous unbroken tube of differential stiffness without abruptjoints. Alternatively, only the ends of the tubing may be formed ofstiff first sections, while a single long soft section may extendtherebetween. The tubing of FIG. 11 may be provided in a long length forcutting at the center of any of its stiffer sections to provide ashorter length of soft, flexible tubing with stiff ends suitable forinserting into quick connect fittings.

FIG. 16 shows another product of the invention, coated guidewire 140 foruse with a medical catheter. Coated guidewire 140 includes guidewire 141jacketed with differential stiffness tubing 142 along part or all of itslength, stiffer proximal section 143, transition section 144, and softerdistal section 145 of tubing 142 providing differential stiffness tocoated guidewire 140. Wire 141 may be a single filament wire. Typically,guidewire 141 has proximal end 146 of uniform stiffness throughout itslength and distal end 147 tapered to decrease its stiffness in thedistal direction. As shown in FIG. 16, jacket 142 may be applied withincreasing thickness in the distal direction to lessen the diameterdifference between the wire proximal end 146 and wire distal end 145,and preferably to provide a uniform or near-uniform outer diameter alongthe length of guidewire 140. The differential stiffness tubing describedherein also may be utilized to jacket a cable to provide differentialstiffness to the cable.

In most of these applications for the invention, the main considerationin the method and systems described herein as well as the specificdesigns of the co-extrusion heads is the ability to make short andcontrolled transition sections in co-extruded tubing which haveinterrupted layers or elements. This technique is thus named "SCTS"technology. We have described in detail how this can be accomplished andwe have also indicated the many and varied applications for a variety ofdifferent types of tubing that one may wish to produce.

The principles of the invention can be used to process a number ofdifferent materials used in making tubing. For example, nylons(polyamides), HDPE'S, polyesters, polypropylenes and other materials,including mineral and fiber-filled materials, can be used for the stifflayer or section of a tubing. For the soft layer or section, suchmaterials as ethylene vinyl acetate, ethylenic copolymers, polyamideelastomers, polyurethanes and other thermoplastic elastomers can beused. If the tubing is for a medical catheter requiring a guidewire,many of the above listed materials for the stiff layer can be used forthe inside layer that will come into contact with the guidewire,especially if the material is combined with current orientationtechnology to provide a low-friction surface. Also, all resins can befilled with radio opacity or not depending upon the intended use of thefinished product. Moreover, in some applications for a finished productmade using the invention principles, good adhesion between layers isnecessary while in other applications that is not a requirement. Ineither case, the invention can be used.

Another advantage of the invention is the versatility that it provides.Because the principles of the invention can be used to produce tubing ina continuous reel, the principles can be combined with othertechnologies to enhance the properties of the finished product. Forexample, tapering combined with lumen air control has been successfullyemployed to vary I.D., O.D. or wall thickness in some sections of acatheter. In particular, the invention can be used to producemicrocatheters with a desired tip section having a thinner wall andsmaller O.D. but with only a very slightly smaller I.D. Braiding ofmetal and non-metal wires can also be used with products produced by theinvention to give the finished product more torqueability, higherstiffness, etc., and wire winding can be added to give additional kinkresistance. Such tubings are illustrated in FIGS. 17 and 18,respectively. FIG. 17, not drawn to scale, shows metal braid reinforcedtubing 150 made up of differential stiffness tubing 151 and metal meshor braid 152 to provide a reinforcing sleeve over tubing 151. In FIG.17, braid 152 is shown as forming a sleeve over only proximal section153 of the tubing. Alternatively, braid 152 may extend distally fromproximal section 153 to provide reinforcement to transition section 154and, if desired, part or all of distal section 155. In FIG. 18, notdrawn to scale, similar features to those shown in FIG. 17 are indicatedby the same reference numerals. FIG. 18 shows wire wound tubing 156 madeup of differential stiffness tubing 151 and metal wire 157 wound aroundtubing 151 to provide reinforcement. In FIG. 18, wire 157 is shown asbeing wound over only proximal section 153 of the tubing. Alternatively,wire 157 may extend distally from proximal section 153 to providereinforcement to transition section 154 and, if desired, part or all ofdistal section 155. Either reinforced tubing 150 or 156 may be, e.g.,heat treated to embed braid 152 or wire 157 in the outer surface of thetube wall, as shown for reinforced tubing 150.

In addition, irradiation and orientation technologies can be employedalong with the invention to produce tubing of higher strength, moredimensional stability, lower elongation, etc. The latter is beneficialto prevent neck-down of catheters that results in clamping of thecatheter onto the guidewire when subjected to axial stress during amedical procedure. Plastic foam technologies can also be employed withthe method of the invention to produce super-soft tips.

From the foregoing description it is obvious that the number of layers,the type of layer and material used for the tubing, etc. will varydepending upon the particular characteristics desired, but it should beunderstood that catheters or tubes having multiple layers of a varietyof materials and arranged differently than the illustrated embodimentscan be formed using the principles of the invention. Although we havedescribed the invention in connection with certain preferred embodimentsthereof, it will be evident to those skilled in the art that variousmodifications can be made to the preferred embodiments and methodsdescribed herein without departing from the spirit and scope of theinvention. It is our intention however, that all such revisions andmodifications that are obvious to those skilled in the art will beincluded within the scope of the following claims:

What is claimed is as follows:
 1. A medical device comprising anelongated tube having a proximal section and a distal section, said tubehaving an annular wall with an outer surface and an inner surface thatdefines a central passageway that extends substantially the length ofthe tube, said tube being constructed of a first material and a secondmaterial having less stiffness than the first material, the wall of thetube gradually changing from the first material to the second materialto form a transition section between the proximal section and the distalsection, the first and second materials being gradually combined in thetransition section and naturally adhering to each other to form a wallgradually changing from one substantially made up of the first materialto one substantially made up of the second material to form a continuousunbroken tube of materials having different properties and withoutabrupt joints, the distal section being made of the second material andtherefore less stiff than the proximal section.
 2. The medical device ofclaim 1 in which the average length of the transition section is about0.25-20 inches.
 3. The medical device of claim 1 in which the proximalsection is substantially longer than the combined length of thetransition section and the distal section, and the transition section isformed into a generally S-shaped configuration.
 4. The medical device ofclaim 1 in which the annular wall defines at least one additionalpassageway extending substantially the entire length of the tube, and aballoon is located at the end of the distal section of the tube, saidballoon adapted to be inflated by fluid passed through one of thepassageways.
 5. The medical device of claim 4 in which the averagelength of the transition section is about 0.25-20 inches.
 6. The medicaldevice of claim 5 further comprising a circumferential layer of braidedmesh material embedded in said wall along at least a portion of thewall.
 7. The medical device of claim 5 further comprising a helicallywound metal coil embedded in said wall in at least a portion of thetube.
 8. The medical device of claim 2 in which the transition sectionis curved in its relaxed state and the distal section is sufficientlysoft to provide a low-trauma end at the end of the distal section forguiding the device when used as a catheter.
 9. The medical device ofclaim 8 in which the transition section is provided with openings toprovide for discharge of fluid passed through the passageway.
 10. Themedical device of claim 2 further comprising a circumferential layer ofbraided mesh material embedded in said wall along at least a portion ofthe wall.
 11. The medical device of claim 2 further comprising ahelically wound metal coil embedded in said wall in at least a portionof the tube.